Crypt backend filename encoding

Feature
7

The problem with base32768 is that Dropbox doesn't support newer unicode code points which seem to be used by b32k. E.g.,

azmi@stiorra:~$ rclone touch azmi-dbx:/good_stuff/foo
azmi@stiorra:~$ rclone ls azmi-dbx:/good_stuff
        0 foo
azmi@stiorra:~$ rclone touch azmi-dbx:/good_stuff/ꡀ
azmi@stiorra:~$ rclone ls azmi-dbx:/good_stuff
        0 foo

I did run the test though, and it seems to have worked?

// crypt-b32k
maxFileLength = 463 // for 1 byte unicode characters
maxFileLength = 231 // for 2 byte unicode characters
maxFileLength = 154 // for 3 byte unicode characters
maxFileLength = 115 // for 4 byte unicode characters